1. Field of the Invention
The present invention relates to an optical pickup apparatus, a reproducing apparatus, a recording apparatus, and a tracking error signal generation method that are suitable for an optical recording medium formed of a plurality of recording layers.
2. Description of the Related Art
An optical disk recording/reproducing apparatus is designed to perform information recording or information reproduction by causing a spot of condensed light beams emitted from a laser light source to follow a track formed in a spiral fashion on an optical recording medium in advance such as an optical disk, for example, a signal track composed of guide grooves or pit arrays. As a method of exercising tracking control for causing a condensed light spot to follow the signal track, in general, a technique of detecting push-pull error signals is adopted (hereafter referred to as “push-pull detection method”).
According to the push-pull detection method, reflected light that has undergone diffraction due to the configuration of guide grooves is detected by using a four-part split light-receiving element having four light-receiving regions obtained by dividing the entire area into two segments both in the direction of the tangent and in the direction of normal to the track of the optical disk, and intensity distribution of interference between zeroth-order diffraction light (hereafter referred to as “main beam”) and ±first-order diffraction light (hereafter referred to as “sub beam”) is detected to thereby generate a tracking error signal.
For example, in a tracking error detecting system of an optical head taken up as a first related art, three pieces of two-part split detectors are used, namely one two-part split detector for detecting a reflection beam obtained as the result of reflection of a main beam from an optical disk (hereafter referred to as “return light”) and two two-part split detectors for detecting return light of a sub beam. A differential output obtained by amplifying a difference between two outputs from the two-part split detector for the main beam in a differential amplifier is corrected with differential outputs obtained by amplifying a difference between two outputs from each of the two-part split detectors for the two sub beams in a corresponding differential amplifier, and is eventually detected as a push-pull error signal, namely a tracking error signal (refer to Japanese Unexamined Patent Publication JP-A 61-94246 (1986), for example).
The method of detecting push-pull error signals adopted in the first related art is a differential push-pull detection method. In this method, a differential push-pull error signal is detected by subtracting, from the differential output detected in respect of the main beam (hereafter referred to as “push-pull signal”), a value obtained by multiplying the sum of push-pull signals detected in respect of the two sub beams by a certain coefficient. By adopting the differential push-pull detection method, it is possible to alleviate the influences of light quantity variation in push-pull error signals, lens offset, and disk tilting.
FIG. 7 is a view showing the configuration of a photodetector 90 in accordance with a second related art. The photodetector 90 of the second related art is composed of a four-part split light-receiving element 901 for detecting return light of a main beam and two two-part split light-receiving elements 902 and 903 for detecting return light of a sub beam. The four-part split light-receiving element 901 is formed of four regions A through D, the two-part split light-receiving element 902 is formed of two regions E and F, and the two-part split light-receiving element 903 is formed of two regions G and H.
In the example shown in FIG. 7, a differential push-pull error signal is detected by subtracting, from a push-pull signal detected in respect of the main beam ‘(output from the region A+output from the region B)−(output from the region C+output from the region D)’, a value obtained by multiplying the sum of push-pull signals detected in respect of the two sub beams (output from the region E−output from the region F) and (output from the region G−output from the region H), namely [(output from the region E−output from the region F)+(output from the region G−output from the region H)] by a certain coefficient.
In general, an optical disk recording/reproducing apparatus using main and sub beams is so designed that the intensity ratio between a main beam and a sub beam is approximately 10:1. In the case of setting the intensity ratio between a main beam and a sub beam to be approximately 10:1, the following problem arises when recording or reproduction is effected on an optical disk having a plurality of recording layers, such as a two-layer disk. For example, in a case where, in a two-layer disk, a target recording layer to be accessed, namely a recording layer for effecting recording or reproduction is irradiated with a condensed light spot and resultant return light is used for signal reproduction, reflected light from the other recording layer which is not an access target (hereafter referred to as “stray light”) is inconveniently detected by a light-receiving element.
Therefore, in the course of differential push-pull error signal generation, in an AGC (Automatic Gain Control) circuit for converting differential push-pull error signals in accordance with disk reflectance and irradiation power developed during recording or reproduction, stray light of the main beam is detected by the light-receiving element for the sub beam. This leads to production of incorrect differential push-pull error signals. In the presence of incorrect differential push-pull error signals, tracking control cannot be exercised in an appropriate manner, which gives rise to a problem of deterioration in servo performance capability.
In the optical disk apparatus taken up as a third related art, in addition to the first light detecting section for detecting reflected light from one of information recording layers, the second light detecting section formed of one or more light-receiving surfaces for detecting stray light from the other information recording layer is mounted on a light-receiving element. Based on the intensity of stray light detected by the second light detecting section, the number of information recording layers laminated on the optical disk is identified. In accordance with the identified information recording layer number, optical pickup control is exercised prior to focus servo control (refer to Japanese Unexamined Patent Publication JP-A 2006-31773, for example).
In the optical head apparatus taken up as the fourth prior art, in addition to the first light-receiving element section for obtaining push-pull signals of zeroth-order diffraction light and the second light-receiving element section for obtaining push-pull signals of ±first-order diffraction light, as the third light-receiving element section, a light-receiving element section for detecting interlayer stray light components, namely stray light components is provided. The first light-receiving element section is a four-part split detecting element, and the second light-receiving element sections, which are two in number, are each a two-part split detecting element. The configurations of the first and second light-receiving element sections are just as in FIG. 7. The third light-receiving element sections are two pairs of detecting elements, of which the detecting elements of each pair are individually placed in the vicinity of either side of their respective two-part split detecting element. In this construction, with use of a signal generated by subtracting, from the sum of signals outputted from the second light-receiving element sections, a signal value obtained by multiplying the detection signal outputted from the third light-receiving element section, namely stray-light component signal by a certain coefficient, stray light components included in the push-pull signal obtained by the second light-receiving element section are removed. In this way, it is possible to diminish the likelihood of calculation of differential push-pull error signals being carried out incorrectly (refer to Japanese Unexamined Patent Publication JP-A 2005-346882, for example).
However, the third related art, although it allows detection of stray light by means of the second light detecting section, is not directed toward adjustment of the influence of detected stray light exerted upon main-beam return light and sub-beam return light.
According to the fourth related art, a differential push-pull error signal, namely a tracking error signal is generated under a condition where interlayer stray light beams are uniform in intensity. That is, the fourth related art is based upon a case where stray light beams having uniform intensity exist on the third light-receiving element section and the second light-receiving element section. In reality, however, the intensity of interlayer stray light is not uniform throughout the region; that is, the stray-light intensity varies from part to part. This makes it impossible to remove stray light components included in sub-beam return light properly, with the result that calculation of differential push-pull error signals, namely tracking error signals is carried out incorrectly.